1. Technical Field of the Invention
The present invention generally relates to a communications system and, in particular, to a digital modem and method that reduces the adverse effects of transmitter non-linear distortion by shifting intermodulation products attributable to the transmitter non-linear distortion off a received signal.
2. Description of Related Art
One of the most significant design challenges for operators in the telecommunications field is to develop new technology or ways of increasing the speed of communications between modems. Recently, the operators have developed and standardized a new modem technology known as Asymmetric Digital Subscriber Line (ADSL) which increases the speed of communications between digital modems. The communications can include multimedia, digital video and high speed data communications.
The ADSL technology effectively converts existing twisted-pair telephone lines into various access paths that enable such communications by increasing the speed at which digital modems communicate with each other. The access paths provided by ADSL include information channels referred to as a downstream channel and an upstream channel, where the downstream channel can overlay the upstream channel. The access paths also include a plain old telephone service (POTS) or an integrated services digital network (ISDN) channel which can work in conjunction with ADSL. The upstream channel and downstream channel can each be submultiplexed to form multiple, lower rate channels.
The current digital multi-tone based duplex communication systems and, more particularly, transmitters in the digital modems create undesirable distortions that have an adverse effect on a signal received by a receiver. The undesirable distortion also referred to as transmitter non-linear distortion is attributable to non-ideal or xe2x80x9creal worldxe2x80x9d components located within the transmitter of the digital modem. For example, such non-ideal components can include an amplifier. To address the transmitter non-linear distortion, care is currently taken in selecting low-distortion components and in designing a receive signal path within the receiver to reduce the adverse effects of the undesirable distortion. Unfortunately, the traditional digital modems are still affected by the transmitter non-linear distortion which is illustrated in greater detail with respect to FIG. 1.
Referring to FIG. 1, there is a frequency spectrum graph illustrating a signal structure from the viewpoint of the traditional digital modem. The signal structure of the traditional digital modem includes a set of discrete tones or carriers at f1, f2 . . . fn and separated by a fundamental frequency (Fb) 102. More specifically, the signal structure includes a transmission band 104 and a receiving band 106 where both bands have a frequency range defined by the ADSL standard. The transmission band 104 includes multiple transmission tones 108 each of which are individually modulated to enable transmission of data to a corresponding traditional digital modem. The receiving band 106 includes multiple receiver tones 110 each of which contains modulated data received from the corresponding traditional digital modem. It should be understood that each receiver tone 110 and each transmission tone 108 are separated by the fundamental frequency.
As mentioned earlier, the transmitter of the traditional digital modem generates the undesirable transmitter non-linear distortion which has an adverse effect on the ability of the receiver to demodulate data received with the receiver tones 110. In a two tone scenario, the transmission non-linear distortion is due to intermodulation products 112 between two carriers f1 and f2 (for example) represented by:
mf1xc2x1nf2xe2x80x83xe2x80x83(1)
where m and n are integers.
In normal operation when all the transmitter tones 108 are considered, the intermodulation products 112, which can have varying signal power levels, appear as discrete carriers spaced apart by multiples of the fundamental frequency 102. The intermodulation products 112 are spread over the transmission band 104 and the receiving band 106. Because some of the intermodulation products 112 fall directly on or very close to the receiver tones 110, this may adversely affect the ability to correctly demodulate the data received from the corresponding traditional digital modem. Moreover, the adverse effect of the intermodulation products 112 is greater when the signal power of the receiver tones 110 is less than the signal power of the transmitter tones 108. Therefore, there is a need for a communications system and, more specifically, a digital modem and method that reduces the adverse effects of intermodulation products by shifting the transmitter tones a predetermined distance to move the intermodulation products off the receiver tones.
The present invention is a communications system, a digital modem and method for reducing non-linear distortion generated by a transmitter which adversely affects a receiver attempting to demodulate received data. More specifically, the digital modem includes a controller that controls a receiver and a transmitter. The receiver is operable to receive a plurality of receiver tones, and the transmitter is operable to generate a plurality of transmitter tones whose intermodulation products (transmitter non-linear distortion) conflict with the plurality of receiver tones. The transmitter is also operable to shift the plurality of transmitter tones by a predetermined distance to move the conflicting intermodulation products off the plurality of receiver tones.